1. Field of the Invention
The present invention relates to a device to measure the gravitational forces in a wellbore and, more particularly, to such a device which is used to measure the gravity difference between two different locations in a wellbore for use in calculating the bulk density of a subterranean formation.
2. Setting of the Invention
For geological exploration and hydrocarbon production purposes it is very helpful to know the bulk density of a subterranean formation. Often, to obtain the bulk density of the formation, a gravitymeter is lowered into a wellbore and a gravity reading is taken at a first location in the wellbore adjacent one part of the formation. The gravitymeter is then lowered and stopped at a second location in the wellbore adjacent the formation. A second gravity reading is taken and the two gravity readings are used to calculate the bulk density of the formation.
The use of conventional gravitymeters to obtain an accurate gravity reading is generally a time consuming and tedious operation which requires a skilled and attentive operator. The operator first levels the gravitymeter within the wellbore and, then, "nulls" the gravitymeter, which means that a gravity sensing device within the gravitymeter is adjusted to be in balance between the downward force of gravity and adjustable counterbalancing force(s). To null the gravitymeter, the operator adjusts the counterbalancing force(s) in response to the movement of a chart recorder at the surface, which indicates the relative position of the gravity sensing device. Once the gravitymeter has been nulled, the operator calculates the gravity reading from the amount of counterbalancing force(s) required to null the gravitymeter. Thereafter, the gravitymeter is raised or lowered to the new location, leveled and nulled, and the operator obtains a second gravity reading. The operator can then calculate the bulk density of the formation.
Other devices have been designed to be used as gravitymeters and one such device is called an accelerometer and consists of two spaced, parallel and horizontal magnets with the same polarity, either North or South, adjacent one another. Spaced between the magnets is a mass which is hingeably connected at one end to the accelerometer's housing to allow for vertical movement of the mass between the magnets. Two spaced horizontal and parallel plates are provided adjacent the magnets and the mass. The plates form part of a capacitive position indicator (CPI) circuit used to sense the relative position of the mass between the plates. The use of CPI circuits to sense the position of a mass, such as in a gravitymeter, is illustrated in "Linearization and Calibration of Electrostatically Feedback Gravity Meters", Moore and Farrell, Journal of Geophysical Research, Vol. 75, No. 5, Feb. 10, 1979, and "Measurements in the Earth Mode Frequency Range by an Electrostatic Sensing and Feedback Gravimeter" Black and Moore, Journal of Geophysical Research, Vol. 71, No. 18, Sept. 15, 1966, which is incorporated herein by reference. The electrical output from the CPI circuit is used to generate a control current which is applied to the mass to create electromagnetic forces to move the mass to a null position between the magnets. These electromagnetic forces move the mass to a central position, where the mass is in balance between the downward force of gravity on the mass and the centralizing electromagnetic forces. A measurement of this control current can be converted into a representation of the gravity at the location in wellbore.
A serious problem with the use of an accelerometer as a borehole gravitymeter is that the equipment utilized to measure the control current does not have the necessary accuracy for a measurement of this control circuit to be converted into a representation of the gravity with the accuracy needed for borehole gravity and bulk density surveys. For example, in measuring the output from an accelerometer the value needs to be measured to about nine (9) significant figures, such as 980000101 microamps. However, the equipment capable of measuring the output has as its best tolerance .+-.100 microamps, which is too wide of a tolerance for use in calculating gravity measurements. In such an example, the current needs to be measured to within .+-.0.5 microamps for use in calculating gravity measurements. To increase the accuracy of the gravity readings from an accelerometer, devices have been developed to generate a constant or bias current, such as a constant current of 980,000,000 microamps, which is introduced into the CPI circuit and the control current; thereby, the resulting control current would not need to be measured to the nine significant figures but only two or three. However, after years of effort and large amounts of money spent to develop such a device to generate the bias current, the devices developed as of this date have an output tolerance of about .+-.20 microamps, which is still too wide for use in gravity and bulk density surveys.
There exists a need for a gradiometer device which does not require leveling and nulling and which produces a highly accurate gravity difference reading, without the problems associated with the devices described above.